Resonators are devices that are often used in radio frequency (RF) circuitry in frequency reference circuits and filter circuits. Generally, resonators need to be high-Q, low loss, stable, have a low temperature coefficient of frequency, have high repeatability, have high accuracy, have a low motional impedance, have a large transduction area, have low electrode resistance, and have high power handling capability. Additionally, RF circuits often require multiple resonators having different resonant frequencies. RF applications, such as RF filters, may span wide ranges of operating frequencies and bandwidths. Resonators having a high electromechanical coupling coefficient may tend to have wide bandwidth.
One common type of resonator is a thin-film bulk acoustic resonator (FBAR), according to the prior art. An FBAR may use a piezoelectric thin-film material surrounded above and below with conductors. Aluminum Nitride may be deposited as a piezoelectric thin-film material; however, the FBAR may predominantly resonate in a thickness mode. Therefore, a resonant frequency of the FBAR may be dependent upon the thickness of a deposited Aluminum Nitride layer, which may be difficult to control. Additionally, a separate Aluminum Nitride layer may be needed for an additional FBAR having a different resonant frequency, which may be limiting or expensive.
An improvement on a traditional FBAR is an FBAR that uses a grown single-crystal FBAR (XBAR), which may have the advantages of good material uniformity, low material defect rates, high material stability, low loss, wide bandwidth, high repeatability, high-Q, and low drift. An alternative to the FBAR is a thin-film piezoelectric-on-substrate resonator (FPOSR), which uses a deposited thin-film, such as Zinc Oxide, on a suspended substrate. The FPOSR may resonate in either a thickness mode or a lateral mode. Lateral vibrations tend to be dependent on the size and shape of the resonator and not dependent upon the thickness of the resonator. However, the deposited thin-film of the FPOSR may have poor frequency stability, high losses, and may have a relatively low electromechanical coupling coefficient due to loading by a substrate layer.
A micro-electro-mechanical systems (MEMS) device includes at least one mechanical element, such as a sensor, an actuator, or a resonator, that is formed using a micromachining process that selectively etches away parts of a wafer. The wafer may include added structural layers and may be made of a semiconductor material, such as Silicon. RF communications systems may use MEMS vibrating structures in MEMS resonators or filter circuits. MEMS resonators may be constructed mechanically to provide excellent acoustic isolation between an anchor and a vibrating structure, which may provide MEMS resonators with a very high-Q. Thus, there is a need for a MEMS resonator which is high-Q, low loss, stable, has a low temperature coefficient of frequency, has a high electromechanical coupling coefficient, has high repeatability, has high accuracy, has a low motional impedance, has a large transduction area, has low electrode resistance, has high power handling capability, and can be produced to address wide ranges of operating frequencies and bandwidths.